asymmetric-key

ASYMMETRIC-KEY(7)         Asymmetric Kernel Key Type         ASYMMETRIC-KEY(7)

NAME
       asymmetric - Kernel key type for holding asymmetric keys

OVERVIEW
       A  kernel  key of asymmetric type acts as a handle to an asymmetric key
       as used for public-key cryptography.  The key material  itself  may  be
       held  inside  the  kernel or it may be held in hardware with operations
       being offloaded.  This prevents direct user access to the cryptographic
       material.

       Keys  may  be  any  asymmetric type (RSA, ECDSA, ...) and may have both
       private and public components present or just the public component.

       Asymmetric keys can be made use of by both the  kernel  and  userspace.
       The  kernel  can make use of them for module signature verification and
       kexec image verification for example.  Userspace is provided with a set
       of  keyctl(KEYCTL_PKEY_*)  calls for querying and using the key.  These
       are wrapped by libkeyutils as functions named keyctl_pkey_*().

       An asymmetric-type key can be loaded by the keyctl utility using a com-
       mand line like:

           openssl x509 -in key.x509 -outform DER |
           keyctl padd asymmetric foo @s

DESCRIPTION
       The  asymmetric-type key can be viewed as a container that comprises of
       a number of components:

       Parsers
              The asymmetric key parsers attempt to identify  the  content  of
              the  payload  blob and extract useful data from it with which to
              instantiate the key.  The parser is only used when  adding,  in-
              stantiating  or  updating  a key and isn't thereafter associated
              with the key.

              Available parsers include ones that can  deal  with  DER-encoded
              X.509, DER-encoded PKCS#8 and DER-encoded TPM-wrapped blobs.

       Public and private keys
              These  are  the  cryptographic  components of the key pair.  The
              public half should always be available,  but  the  private  half
              might  not be.  What operations are available can be queried, as
              can the size of the key.  The key material may or may not  actu-
              ally reside in the kernel.

       Identifiers
              In  addition  to the normal key description (which can be gener-
              ated by the parser), a number of supplementary  identifiers  may
              be  available  that can be searched for.  These may be obtained,
              for example, by hashing the public key material or from the sub-
              jectKeyIdentifier in an X.509 certificate.

              Identifier-based  searches  are  selected  by passing as the de-
              scription to keyctl_search() a string constructed of hex charac-
              ters  prefixed with either "id:" or "ex:".  The "id:" prefix in-
              dicates that a partial tail match is permissible  whereas  "ex:"
              requires  an exact match on the full string.  The hex characters
              indicate the data to match.

       Subtype
              This is the driver inside the kernel that accesses the key mate-
              rial  and performs operations on it.  It might be entirely soft-
              ware-based or it may offload the operations to  a  hardware  key
              store, such as a TPM.

       Note  that  expiry  times from the payload are ignored as these patches
       may be used during boot before the system clock is set.

PARSERS
       The asymmetric key parsers can handle keys in a number of forms:

       X.509  DER-encoded X.509 certificates can be accepted.  Two identifiers
              are constructed: one from from the certificate issuer and serial
              number and the other from the subjectKeyIdentifier, if  present.
              If  left  blank,  the key description will be filled in from the
              subject field plus either the subjectKeyIdentifier or the  seri-
              alNumber.  Only the public key is filled in and only the encrypt
              and verify operations are supported.

              The signature on the X.509 certificate may  be  checked  by  the
              keyring  it is being added to and it may also be rejected if the
              key is blacklisted.

       PKCS#8 Unencrypted DER-encoded PKCS#8 key data containers  can  be  ac-
              cepted.   Currently no identifiers are constructed.  The private
              key and the public key are loaded from the  PKCS#8  blobs.   En-
              crypted PKCS#8 is not currently supported.

       TPM-Wrapped keys
              DER-encoded  TPM-wrapped  TSS  key  blobs can be accepted.  Cur-
              rently no identifiers are constructed.  The public  key  is  ex-
              tracted from the blob but the private key is expected to be res-
              ident in the TPM.  Encryption and signature verification is done
              in software, but decryption and signing are offloaded to the TPM
              so as not to expose the private key.

              This parser only supports TPM-1.2 wrappings and enc=pkcs1 encod-
              ing  type.   It  also uses a hard-coded null SRK password; pass-
              word-protected SRKs are not yet supported.

USERSPACE API
       In addition  to  the  standard  keyutils  library  functions,  such  as
       keyctl_update(),  there  are  five calls specific to the asymmetric key
       type (though they are open to being used by other key types also):

              keyctl_pkey_query()
              keyctl_pkey_encrypt()
              keyctl_pkey_decrypt()
              keyctl_pkey_sign()
              keyctl_pkey_verify()

       The query function can be used to retrieve information about  an  asym-
       metric  key, such as the key size, the amount of space required by buf-
       fers for the other operations and which operations  are  actually  sup-
       ported.

       The  other operations form two pairs: encrypt/decrypt and create/verify
       signature.  Not all of these operations will necessarily be  available;
       typically,  encrypt and verify only require the public key to be avail-
       able whereas decrypt and sign require the private key as well.

       All of these operations take an information string parameter that  sup-
       plies  additional  information such as encoding type/form and the pass-
       word(s) needed to unlock/unwrap the key.  This  takes  the  form  of  a
       comma-separated list of "key[=value]" pairs, the exact set of which de-
       pends on the subtype driver used by a particular key.

       Available parameters include:

       enc=<type>
              The encoding type for use in an encrypted blob or  a  signature.
              An example might be "enc=pkcs1".

       hash=<name>
              The  name of the hash algorithm that was used to digest the data
              to be signed.  Note that this is only used to construct any  en-
              coding  that  is  used in a signature.  The data to be signed or
              verified must have been parsed by the caller and the hash passed
              to  keyctl_pkey_sign()  or  keyctl_pkey_verify() beforehand.  An
              example might be "hash=sha256".

       Note that not all parameters are used by all subtypes.

RESTRICTED KEYRINGS
       An additional keyutils function, keyctl_restrict_keyring(), can be used
       to  gate  a keyring so that a new key can only be added to the affected
       keyring if (a) it's an asymmetric key, (b) it's validly signed by a key
       in some appropriate keyring and (c) it's not blacklisted.

            keyctl_restrict_keyring(keyring, "asymmetric",
                                    "key_or_keyring:<signing-key>[:chain]");

       Where  <signing-key>  is  the ID of a key or a ring of keys that act as
       the authority to permit a new key to be  added  to  the  keyring.   The
       chain  flag indicates that keys that have been added to the keyring may
       also be used to verify new keys.  Authorising keys must  themselves  be
       asymmetric-type keys that can be used to do a signature verification on
       the key being added.

       Note that there are various system keyrings visible to  the  root  user
       that may permit additional keys to be added.  These are typically gated
       by keys that already exist, preventing  unauthorised  keys  from  being
       used for such things as module verification.

BLACKLISTING
       When the attempt is made to add a key to the kernel, a hash of the pub-
       lic key is checked against the blacklist.  This  is  a  system  keyring
       named .blacklist and contains keys of type blacklist.  If the blacklist
       contains a key whose description matches the hash of the new key,  that
       new key will be rejected with error EKEYREJECTED.

       The  blacklist keyring may be loaded from multiple sources, including a
       list compiled into the kernel  and  the  UEFI  dbx  variable.   Further
       hashes  may also be blacklisted by the administrator.  Note that black-
       listing is not retroactive, so an asymmetric key that is already on the
       system  cannot  be  blacklisted  by  adding  a matching blacklist entry
       later.

VERSIONS
       The asymmetric key type first appeared in v3.7 of the Linux kernel, the
       restriction function in v4.11 and the public key operations in v4.20.

SEE ALSO
       keyctl(1), add_key(2), keyctl(3), keyctl_pkey_encrypt(3),
       keyctl_pkey_query(3), keyctl_pkey_sign(3), keyrings(7), keyutils(7)

Linux                             8 Nov 2018                 ASYMMETRIC-KEY(7)